Industrial applications of the air direct-contact,
gravel, ground heat exchanger
Wojciech Cepiński1,*, Maciej Besler1,†
1Wrocław University of Science and Technology, Faculty of Environmental Engineering, Department of Air Conditioning, Heating, Gas Engineering and Air Protection, ul. Norwida 4/6, 50-373 Wrocław, Poland
Abstract. The paper describes the analysis of possibility of using the air direct-contact, gravel, ground heat exchanger (Polish acronym BGWCiM), patented at the Wroclaw University of Science and Technology to prepare air for conditioning rooms in the industry. Indicated the industry sectors where the application may be the most beneficial.
1 Introduction
1.1 Reasons for raising the issue
The urgent need to save energy obliges to more widespread use of solutions enhancing generation of natural energy which exists in nature and is easily accessible. One of methods to use it is the air direct-contact, gravel, ground heat ex-changer (Polish acronym BGWCiM), tested thoroughly at the Wroclaw University of Science and Technology and the advantages of which will be presented. Industry is one of the most important sectors of a national economy. It is worth mentioning that together with agriculture and construction, industry consumes nearly half of all energy produced. Heavy industry has got an important share. In the processing industry, most of the energy is consumed by the food, chemical, mineral, steel and paper industries [7]. Greater efficiency in the use of energy in the economy, and especially in the industrial sector, has got a significant impact on production costs, profits of enterprises, as well as competitiveness of their products on the world market. It also contributes to faster development of enterprises.
1.2 The air direct-contact, gravel, ground heat exchanger
For many years, research has been carried out at the Institute of Air Conditioning and District Heating of the Wroclaw University of Science and Technology on efficient capture of natural heat (and also cooling) from a small depth of ground, in the air direct-contact, gravel, ground heat exchanger for ventilation and air conditioning purposes [1–3].
In the exchanger, the outside air is led horizontally through a 3–5 m long accumulation bed (fig. 1) [1]. Following the contact of the air flowing between the filling of the bed, its temperature is brought closer to the filling’s temperature, which during peak load results in heating of the outside air in winter from -18°C to about 0°C, and cooling it in summer from +30°C to about +20°C.
Fig. 1. Various proposals for the exchanger construction: A – recessed, B – partially recessed, C – elevated above the level of the existing terrain and situated on the slope, 1 – accumulation bed, 2 – distribution channel, 3 – collecting channel, 4 – thermal/humidity insulation, 5 – exchanger cover, 6 – air intake, 7 – distribution bed, 8 – collecting bed, 9 – natural ground.
Experience has shown that the accumulation bed should be made of washed gravel, gravel or rock grit having a hydraulic diameter of 15–40 mm but without fractions smaller than 3 mm. Stones having a hydraulic diameter of 80–100 mm constitutes a distribution layer. Thermal/humidity insulation should be made of 10 cm polystyrene protected with two layers of double foil. The entire area should be covered with about a half-meter layer of soil in order to enable planting of shrubs or lawn cultivation.
In the exchanger, the outside air is led horizontally through a 3–5 m long accumulation bed (fig. 1) [1]. Following the contact of the air flowing between the filling of the bed, its temperature is brought closer to the filling’s temperature, which during peak load results in heating of the outside air in winter from -18°C to about 0°C, and cooling it in summer from +30°C to about +20°C.
Fig. 1. Various proposals for the exchanger construction: A – recessed, B – partially recessed, C – elevated above the level of the existing terrain and situated on the slope, 1 – accumulation bed, 2 – distribution channel, 3 – collecting channel, 4 – thermal/humidity insulation, 5 – exchanger cover, 6 – air intake, 7 – distribution bed, 8 – collecting bed, 9 – natural ground.
Experience has shown that the accumulation bed should be made of washed gravel, gravel or rock grit having a hydraulic diameter of 15–40 mm but without fractions smaller than 3 mm. Stones having a hydraulic diameter of 80–100 mm constitutes a distribution layer. Thermal/humidity insulation should be made of 10 cm polystyrene protected with two layers of double foil. The entire area should be covered with about a half-meter layer of soil in order to enable planting of shrubs or lawn cultivation.
The heat exchanger (distribution and accumulation beds) should not exceed 5 m length and 2.5 m height. Studies have shown that continuous operation of a 5 m long bed during a summer day results in an optimal shift of a heat wave in the filling. As a consequence,
lowest temperatures of the air leaving the exchanger are achieved at the highest outdoor temperatures [1, 2].
The nominal air velocity, depending on operating mode, may range from 0.05 to 0.20 m/s. Such values result from the heat and mass exchange conditions be-tween the air and the filling, as well as from resistance of the airflow across the exchanger.
A sprinkler system can be performed under the layer of thermal/humidity insulation. Based on a study [1], it was found that sprinkler pipes having a nominal diameter of ø 25 mm, perforated with ø 2 mm holes made at an angle of 90° at intervals of 10 cm are the cheapest and best for this purpose. The optimal distance between pipes is 0.5 m. The sprinkler system enables possible periodic disinfection and washing of the bed. It also facilitates intensification of heat and mass exchange processes in the accumulation filling, especially in the context of increasing the relative humidity of the air which leaves it. The research conducted by the Sanitary-Epidemiology Station in Wroclaw on an exchanger which has been working for many years without sprinkling did not show any microorganisms harmful to human health in the air discharged from the exchanger. On contrary – it was concluded that the number of particles from air pollution following the exchanger got reduced to half of the value contained in atmospheric air [2]. Thus, the exchanger is additionally a specific kind of filter. These results are also confirmed by publications concerning other exchangers of this type operating in Poland. The experiments [2] of hydrating of the bed during its operation show that it does not cause a significant decrease in the air temperature. It only, due to moisturising, significantly increases relative humidity to values near to saturation, i.e. φ = 100%. Additionally, during the course of the study, it was noted that similar parameters are obtained by humidifying only the inlet section of the filling [2]. This allows for possible savings when constructing systems where it is important for some operating periods to achieve higher relative air humidity at the outlet. In addition, what is important, hydraulics tests of the filling performed during sprinkling of the inlet section only, as well as of the entire filling, did not show increase in resistance of the airflow across the bed.
Fig. 2. Practical chart to determine the pressure loss in the bed according to [3].
Fig. 2 shows a practical, usable during designing, experimentally determined dependency between air flow velocity (v) and pressure loss (dp) of the accumulation bed for different hydraulic diameters used in BGWCiM.
-20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 32 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 external temperature, °C tem pera tu re, °C 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 tim e , h after BGWCiM, °C external temperature, °C time, h
Fig. 3. Average air temperature values following the ground exchanger and a number of hours of occurrence of given outdoor temperatures during a year for A type of heat exchanger, 5 m length, 0.14 m/s air velocity, operated year-round continuously [2, 3].
In fig. 3 average values of air temperature after passing through BGWCiM and average times of occurrence of given outdoor temperature data for Wroclaw [2] were presented. They were indicated and statistically calculated on the basis of many years of measurements. This chart can be used in designing and provide the basis for performance of any analyses connected with the full-year operation of the device.
As a result of air contact with the ground in BGWCiM, the soil gets drained during periods of hot summer, moisturised in winter and cleansed. The above is achieved only by negligible energy inputs necessary to overcome the airflow resistance through the bed.
2 Conditioning rooms in the industry
2.1 Required air parameters
-20 -16 -12 -8 -4 0 4 8 12 16 20 24 28 32 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 external temperature, °C tem pera tu re, °C 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 tim e , h after BGWCiM, °C external temperature, °C time, h
Fig. 3. Average air temperature values following the ground exchanger and a number of hours of occurrence of given outdoor temperatures during a year for A type of heat exchanger, 5 m length, 0.14 m/s air velocity, operated year-round continuously [2, 3].
In fig. 3 average values of air temperature after passing through BGWCiM and average times of occurrence of given outdoor temperature data for Wroclaw [2] were presented. They were indicated and statistically calculated on the basis of many years of measurements. This chart can be used in designing and provide the basis for performance of any analyses connected with the full-year operation of the device.
As a result of air contact with the ground in BGWCiM, the soil gets drained during periods of hot summer, moisturised in winter and cleansed. The above is achieved only by negligible energy inputs necessary to overcome the airflow resistance through the bed.
2 Conditioning rooms in the industry
2.1 Required air parameters
Each type of production, depending on the technology used, requires very specific microclimate conditions (table 1) to produce a given product of possibly the highest quality. What must not be overlooked is also the human aspect. In order to achieve high productivity, the best possible conditions must also be created in the work area. It often happens that the production requirements and the comfort conditions of the people differ. It is when the technology is more important. Ventilation in an industrial plant constitutes a service component rather than a core business. However, production would be impossible without the optimal conditions of the microclimate. An important issue for the ventilation system to be economically viable. Therefore, it must provide appropriate parameters for both the technological process and people, ensuring minimal investment and operating costs [4].
Table 1. Recommended temperature and humidity ranges for various industrial processes [6].
No. Industry Plant type Temperature Relative humidity
- - - C %
1 2 3 4 5
1 Libraries Books warehouse 21–25 40–50
Reading rooms 21–25 35–55
2 Breweries Fermentation rooms 4–8 60–70
Barrels of malt 10–15 80–85 3 Printing works Paper warehousing 20–26 50–60
Printing 22–26 45–60
Multicolour printing 24–28 46–50 Photographic printing 21–23 60
All other works 21–23 50–60
4 Furs Warehousing 5–10 50–60
Drying 43 -
5 Museums Paintings 18–24 40–55
6 Bakeries Flour warehouse 15–25 50–80
Yeast warehouse 0–5 60–75
Dough making 23–25 50–60
Sugar warehouse 25 35
7 Mushroom cultivation Growth period 10–18 -
Warehousing 0–2 80–85
8 Linoleum production Oxidation of linseed oil 32–38 20–28
Printing 26–28 30–50
9 Confectionery Warehousing (dried fruit) 10–13 50
industry Soft sweets 21–24 45
Production of hard sweets 24–26 30–40 Packing of hard sweets 24–28 40–45 Chocolate production 15–18 50–55 Chocolate wrapping 24–27 55–60
Chocolate packing 18 55
Chocolate warehousing 18–21 60–65 Fruitcake and wafers production 18–20 50 10 Electrical engineering General production 21 50–55
industry Production of thermo- and, hygrostats 24 56–55 Production with small tolerances 22 40–45 Production of isolations 24 65–70 11 Pharmaceutical Semi-products warehousing 21–27 30–40 industry Manufacture of tablets 21–27 35–50 12 Photographic Production of normal films 20–24 40–65 industry Production of non-flammable films 15–20 45–50 Treatment of films 20–24 40–60
Darkroom 21–22 45–50
Photographic studio 22–23 40–50 Films warehousing 18–22 40–60
13 Rubber industry Warehousing 16,24 40–50
Manufacturing 31–33 -
Vulcanisation 26,28 25–30
Surgical materials 24–33 25–30
14 Paper Machines 22–30 50–60
industry Paper warehouses 20–24 50–60
15 Tobacco industry Raw tobacco warehousing 21–23 50,65 Tobacco preparation 22–26 75–85 Tobacco warehousing 24–25 70–75
Tobacco steeping 32 85–88
Manufacture of cigarettes and cigars 21–24 55–65
Table 1. continued…
No. Industry Plant type Temperature Relative humidity
- - - C %
1 2 3 4 5
16 Textile industry Bath 22–25 40–50
- cotton Carding machine 22,25 50–55
Comber 22–25 45–55
Textile stretching machine 22–25 50–55
Roving frame 22–25 50–55
Ring frame 22–25 40–45
Spooling, spinning, trimming,
pulling warp 22–25 60–70
Textile industry Preparation 18–20 80
- flax fibres Carding mill 20–25 50–65
Spinning 24–27 65–75
Weaving 27 65–70
Textile industry Preparation 27–29 60
- wool Carding mill 27–29 65–70
Spinning 27–29 50–60
Weaving 27–29 60–70
Equipping 24 50–60
Textile industry Preparation 27 60–65
- silk Spinning 24–27 65–70
Weaving 24–27 60–75
Textile industry Carding, spinning 21–25 65–75
- rayon Weaving 24–25 60–65
17 Match Production 18–22 50
industry Warehousing 15 50
18 Mechanical Offices, warehousing, general assembly 20–24 35–55
plants Precision assembly 22–24 40–50
19 Matches Production 22–23 50
Drying 21–24 60
Warehousing 16–17 50
20 Leather Drying 20–52 75
Warehousing 10–18 40–60
21 Plastics Thermosetting injection-moulded plastics 27 25–30 Cellophane coated 24–27 45–65
22 Optical industry Application 24 45
Grinding 27 80
23 Ceramics Treatment of refractory materials 43–66 50–90
Storage of clay 16–27 35–63
Decoration of ceramics 24–27 48
24 Electronics Winding of reels 22 15
Semiconductor assemblies 20 40–60 25 Electrical devices Production and metrological tests 21 50–55 Assembly and calibration of thermostats 24 50–55 Assembly and calibration of hygrostats 24 50–55 26 Precision mechanics Assemblies having strict dimension tolerances 22 40–45 Assembly and tests of measuring devices 24 60–63 27 Distributor apparatus Fuses and circuit breakers 23 50
and switchers Winding of power capacitors 23 50 Paper insulation warehousing 23 50
2.2 Use of BGWCIM
Table 1. continued…
No. Industry Plant type Temperature Relative humidity
- - - C %
1 2 3 4 5
16 Textile industry Bath 22–25 40–50
- cotton Carding machine 22,25 50–55
Comber 22–25 45–55
Textile stretching machine 22–25 50–55
Roving frame 22–25 50–55
Ring frame 22–25 40–45
Spooling, spinning, trimming,
pulling warp 22–25 60–70
Textile industry Preparation 18–20 80
- flax fibres Carding mill 20–25 50–65
Spinning 24–27 65–75
Weaving 27 65–70
Textile industry Preparation 27–29 60
- wool Carding mill 27–29 65–70
Spinning 27–29 50–60
Weaving 27–29 60–70
Equipping 24 50–60
Textile industry Preparation 27 60–65
- silk Spinning 24–27 65–70
Weaving 24–27 60–75
Textile industry Carding, spinning 21–25 65–75
- rayon Weaving 24–25 60–65
17 Match Production 18–22 50
industry Warehousing 15 50
18 Mechanical Offices, warehousing, general assembly 20–24 35–55
plants Precision assembly 22–24 40–50
19 Matches Production 22–23 50
Drying 21–24 60
Warehousing 16–17 50
20 Leather Drying 20–52 75
Warehousing 10–18 40–60
21 Plastics Thermosetting injection-moulded plastics 27 25–30 Cellophane coated 24–27 45–65
22 Optical industry Application 24 45
Grinding 27 80
23 Ceramics Treatment of refractory materials 43–66 50–90
Storage of clay 16–27 35–63
Decoration of ceramics 24–27 48
24 Electronics Winding of reels 22 15
Semiconductor assemblies 20 40–60 25 Electrical devices Production and metrological tests 21 50–55 Assembly and calibration of thermostats 24 50–55 Assembly and calibration of hygrostats 24 50–55 26 Precision mechanics Assemblies having strict dimension tolerances 22 40–45 Assembly and tests of measuring devices 24 60–63 27 Distributor apparatus Fuses and circuit breakers 23 50
and switchers Winding of power capacitors 23 50 Paper insulation warehousing 23 50
2.2 Use of BGWCIM
In fig. 4 there are areas presented which show the nature of the variability of the outdoor air and the air following BGWCiM, recorded in the climatic zone of Poland.
Moreover, in this drawing the so-called climatic curve is indicated, which shows average outdoor air parameters of Poland’s geographical area and typical parameters of thermal comfort.
When referring to the air parameters following BGWCiM to the air parameters required in production processes (room temperature and air conditioning temperature), it can be seen that the use of the exchanger for air preheating will not always be cost effective. During the cold season, except for processes requiring very low humidity in the room (dry rooms), preheating and humidification will be almost always beneficial. This differs in the case of the transition and warm periods, when too much humidification of the air in the ground heat exchanger, usually close to the saturation parameters, will cause the need to dry it, which will be associated with additional costs which can exceed the savings resulting from the temperature drop.
Beneficial effects of using BGWCiM should be expected in air treatment systems for air conditioning of rooms with relatively high temperature and high relative humidity (e.g. textile industry, tobacco industry, etc.).
Mollier H-x chart for humid air (b = 0.1 MPa)
2 4 6 8 10 0 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 29 30 28 27 26 24 23 22 21 19 18 17 16 14 13 12 11 9 8 7 10 15 25 5 6 20 4 3 2 1
wilgotność właściwa g/kg pow. suchego 0 1,34 1,36 1,32 1,30 1,28 1,26 1,24 1,22 1,20 1,18 1,16 1,14 1,12 1,10 1,08 1,06 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 5 tem pe ra tur a t erm om etru su ch eg o °C gę sto ść kg/ m 3 wilgotność względna % 10 15 20 25 30 40 50 60 70 80 90 100
ciśnienie cząstkowe pary wodnej mbar
tempera tura term ometru mokre go °C 100 80 60 40 30 20 10 0 90 70 50 -10 -20 entalp ia kJ/kg 30 25 20 15 10 5 +0 -0 -5 -10 -15 -20 2224 2628 3234 3638 4244 4648 5254 5658 6264 6668 7274 7678 828486 88 92 94 9698 18 16 14 12 8 6 4 2 -2 -4 -6 -8 -12 -14 -16 -18 wsp ółczy nn ik ką to wy p rz em ian y kJ /kg +? -? 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 30 000 20 00 0 10000 8000 7000 6000 5000 4000 3000 2000 1000 ±0 -1000-2000-3000-4000 -5000 -10000-20000-30000
microclimate conditions
for home ventilation
variability
of the outdoor air
climatic curve
variability air
following BGWCiM
average values
of air temperature
after BGWCiM
3 Conclusion
The decision to use BGWCiM to pre-prepare the air in the process of air conditioning of production rooms should be each time preceded by an in-depth analysis of the full-year operation of the system with a full assessment of possible effects. Restrictive requirements for microclimate parameters in many industries will not always be achievable with BGWCiM alone or with BGWCiM serving to pre-prepare the air. Wherever the analysis confirms the suitability of its use, savings of up to 30% per year of energy consumed can to be expected, compared to BGWCiM-free system while reducing power and size of other air treatment devices (e.g. heaters, radiators, heat sources, chillers, humidifiers, etc.) [2].
References
1. G.J. Besler and others, The air direct-contact, gravel, ground heat exchanger, Patent No. 128261 PRL, Wroclaw University of Science and Technology, Wrocław 1980
(in Polish).
2. M. Besler, Research on energy efficiency of soil in the environmental engineering, PhD thesis, Wrocław 1997 (in Polish)
3. W. Cepiński, Efficiency of natural energy acquisition, PhD thesis, Wrocław 2010 (in Polish)
4. A. Pełech, Ventilation and air conditioning. Fundamentals, Publishing House of Wroclaw University of Science and Technology, Wrocław 2008 (in Polish)
5. S. Przydrożny, Ventilation, Publishing House of Wroclaw University of Science and Technology, Wrocław 1991 (in Polish)
6. A. Słomka, Ventilation and air conditioning for work rooms Library of the PIP Training Center, 2007(in Polish)
